Rupture dynamics and arrest of 2015 Mw 7.8 Gorkha earthquake

Abstract:

2015 Mw 7.8 Gorkha earthquake ruptures the main Himalayan thrust (MHT) fault, along which the Indian plate is subducting beneath the Eurasian plate. It propagates about 150 kms ESE-ward along strike rupturing downdip edge of the seismogenic zone, only to stop abruptly to the east. Co-seismic rupture and aftershock patterns show considerable complexity both along strike and dip. The controlling factors of such complexities and abrupt rupture arrest, however, remain poorly understood. Here, we use multiple independent seismic techniques to reveal the details of the complexities and better understand structural control over rupture dynamics.

We have used a local dense seismic network NAMASTE (Karplus et al., 2020) to capture a prolific sequence of aftershock activity in high resolution. It illuminates a duplex structure that produces majority of the aftershocks in this sequence (Mendoza et al., 2019). The aftershocks also show spatially heterogeneous pattern with eastern part of the rupture area being much more active than western. Interestingly, seismicity in the east is deeper than west with a relatively sharp shift in depth of about 4 kms in between. P-wave velocity changes from local earthquake tomography is consistent with this depth shift. This area also coincides with anomalously high b-value and low P-wave velocity. Backprojection results using multiple teleseismic arrays indicates that rupture radiates peak high-frequency seismic energy in this area and starts to separate along-dip bifurcating into two branches. A northern and a southern branch of the rupture wrap around an area that eventually fails to produce the largest aftershock of the Gorkha event. Depth shift in seismicity is likely cause by a lateral ramp resulting in different behavior in eastern and western part of the rupture area.

The arrest of the Gorkha rupture to the east is characterized by an abrupt truncation of aftershock activity, producing a remarkably sharp seismic lineation. Close inspection of aftershock distribution reveals multiple well-defined steeply-dipping faults at depth. Interestingly, this area also hosts a number of notable transverse faults identified at the surface such as, Gaurishankar fault, Evevest fault, and Patna fault. These faults may act as barriers to the earthquake propagations at depth, and abruptly stop runaway rupture propagation of Gorkha earthquake farther to east limiting its size. Such transverse faults may define segment boundaries determining the size of the damaging Himalayan earthquakes.